Two questions, one about defensive pacts and another about city bombardment:
I can't remember if this is the way Defensive Pacts always worked, or if it was intentionally changed in the mod, but here's what happened:
I (Japan + vassals) have defensive pacts with England, Colombia, and Brazil. I want to declare war on Spain, who is in a defensive pact with France. I previously thought that being the aggressor instantly dissolved your own defensive pacts, but in this case I declared war on Spain, which prompted France to declare war on me, and then unexpectedly England, Colombia, and Brazil all declared war on France and Spain as well. This seems a bit counter-intuitive to me because it means I can intentionally pull a whole bunch of civs into wars as long as I have a target with a defensive pact, but against a target without any defensive pacts they would stay out of it, and the same would apply to me. For example, England could've declared war on France, prompting Spain to declare on England, which would then bring in my Japan. That sounds a lot more like a broad military alliance, not just a defensive pact.
Secondly, and perhaps this is just an unfortunate side effect of allowing cities to be spaced more closely, a siege unit placed adjacent to two cities will reduce both cities' defenses when ordered to bombard.